π-Delocalization in Oligoalkynes Induced by Push−Pull Substituents and 1,3-Conjugation: A Combined13C NMR and Computational Study

Abstract

(13)C chemical shifts of the push-pull oligoalkynes Don-(C identical withC)(n)-Acc (n = 1-4; Don = morpholino; Acc = COMe, COOMe) were computed at the DFT (B3LYP/6-311+G(d,p) level of theory compared with the experimental delta values and the agreement employed as a measure of quality for the underlying structures. For the global minima structures, the occupation quotients of antibonding pi* and bonding pi orbitals (pi*(C[triple bond]C)/pi(C[triple bond]C)) and the bond lengths (d(C[triple bond]C)) of the various C[triple bond]C triple bonds were also computed and correlated to each other. The linear dependence obtained for the two parameters d(C identical withC) and pi*(C[triple bond]C)/pi(C[triple bond]C) quantifies changes in pi-delocalization induced by the push-pull effect of the substituents and 1,3-conjugation (1,3,5- and 1,3,5,7-, respectively) of the C identical withC bonds in the oligoalkynes studied. A critical comparison of the push-pull effect, attenuated with increasing n, and the conjugative stabilization of the oligoalkynes, increasing with n, as concluded from d(C identical withC) and pi*(C[triple bond]C)/pi(C[triple bond]C) of the oligoalkynes and the reference compounds Me-(C[triple bond]C)(n)-Me, Don-(C[triple bond]C)(n)-Me, and Me-(C[triple bond]C)(n)-Acc), respectively (Don = morpholino; Acc = COMe, COOMe), is affiliated.